Technique for comparing analog signal with reference voltage

ABSTRACT

The present invention provides a technique for reducing power consumption of an A/D converting device. An A/D converting device according to the present invention includes: multiple comparators each of which has a function for comparing input analog signals with a predetermined reference voltage; an A/D converter for converting the input analog signal into the digital signal with a predetermined number of bits using comparison results from the multiple comparators; a determining unit for determining which range the voltage of the input analog signal belongs to before the comparators comparing the input analog signal with the reference voltages; and a control unit for controlling power supply for the multiple comparators corresponding to the range determined by the determining unit. Thus, power is supplied only to the comparators necessary for this particular A/D conversion, thereby reducing power consumption.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for processing analogsignals, and particularly to a comparing device for comparing analogsignals with a reference voltage, a method thereof, an analog/digitalconverting device for converting analog signals into digital signalsusing the comparing device, and a determining device used for thecomparing device.

2. Description of the Related Art

As an example of a circuit for converting input analog signals intodigital signals, a pipeline A/D converter is known (e.g., JapaneseLaid-Open Patent Publication No. 9-275342). The pipeline A/D converteris formed of multiple low-bit sub-A/D converters connected one toanother. With such a configuration, the pipeline A/D converter performsA/D conversion in steps, by actions of each sub-A/D converter. Eachsub-A/D converter includes multiple comparators for comparing inputanalog signals with a reference voltage, whereby the analog signals areconverted into digital signals.

In recent years, rapidly advancing fields such as image processing,communication, and so forth, require a technique for processing digitalsignals with higher precision at a higher speed. Accordingly,development of an A/D converter for performing high-speed and multi-bitA/D conversion is being undertaken. However, a high-speed A/D converterrequires high-speed comparators forming the A/D converter, leading toincreased power consumption of the comparators. Furthermore, in general,such a multi-bit A/D converter requires a greater number of comparators.Accordingly, realizing such a high-speed multi-bit A/D converter demandsdeveloping of a technique which enables effective comparison with assmall a number of comparators as possible while reducing powerconsumption of each comparator.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, andaccordingly, it is an object thereof to provide a technique for reducingpower consumption of a comparing device or an analog/digital convertingdevice.

A first aspect of the present invention relates to a comparing device.The comparing device comprises multiple comparators for comparing inputanalog signals with predetermined reference voltages, with theconceivable range of the voltage of the input analog signal beingdivided into multiple ranges with predetermined thresholds asboundaries, and with power supply for the multiple comparators beingcontrolled corresponding to the range which the voltage of the analogsignal belongs to. For example, with a configuration wherein theconceivable range of the voltage of the input analog signal is dividedinto two ranges, the median of the maximum value and the minimum valueof the input analog signal may be employed as the threshold. Such aconfiguration enables suitable power control such as reduction of powersupply for the comparators which are not required for this particularA/D conversion, for example. Thus, such a configuration enablesreduction of power consumption of the comparing device.

The comparing device may further comprise: a determining unit fordetermining which range the voltage of the input analog signal belongsto; and a control unit for controlling power supply for the multiplecomparators corresponding to the range determined by the determiningunit, with the control unit reducing power supply for the comparatorseach of which receives a reference voltage outside of the range. Such aconfiguration reduces power supply for the comparators whereincomparison results are already obvious, thereby reducing powerconsumption of the comparing device.

A second aspect of the present invention relates to a comparing device.The comparing device comprises multiple comparators for comparing inputanalog signals with predetermined reference voltages, with theconceivable range of the voltage of the input analog signal beingdivided into multiple ranges with predetermined thresholds asboundaries, and with the reference voltages, which are to be input tothe multiple comparators, being adjusted corresponding to the rangewhich the voltage of the analog signal belongs to. Such a configurationallows adjustment of the reference voltages corresponding to the rangeof the voltage of the input analog signal, thereby appropriatelycomparing analog signals with a wider range with a smaller number ofcomparators. This contributes to reduction of the size of the device,the weight thereof, the cost thereof, and so forth, while reducing thenumber of the comparators and power consumption.

The comparing device may further comprise: a determining unit fordetermining which range the voltage of the input analog signal belongsto; and a switching unit for changing the reference voltages input tothe multiple comparators corresponding to the range determined by thedetermining unit, with the switching unit changing the referencevoltages such that the range between the maximum value and the minimumvalue of the reference voltages input to the multiple comparators coversthe range determined by the determining unit. This allows appropriatecomparison of analog signals using a smaller number of comparators.

Further, a third aspect of the present invention relates to ananalog/digital converting device. The analog/digital converting devicecomprises: an A/D converter including multiple comparators each of whichcompares input analog signals with a predetermined reference voltage,for converting the input analog signal into the digital signal with apredetermined number of bits using comparison results from the multiplecomparators; a determining unit for determining which range the voltageof the input analog signal belongs to, before the comparators comparingthe input analog signal with the reference voltages, with regard tomultiple ranges into which the conceivable range of the voltage of theinput analog signal is divided with predetermined thresholds asboundaries; and a control unit for controlling power supply for themultiple comparators corresponding to the range determined by thedetermining unit.

An arrangement may be made wherein the aforementioned control unitreduces power supply for the comparators each of which receives areference voltage outside of the range thus determined. Also, anarrangement may be made wherein the aforementioned control unit stopspower supply for the comparators each of which receives a referencevoltage outside of the range thus determined. Such a configurationprevent useless power consumption for the comparators which are notrequired for this particular A/D conversion, thereby reducing powerconsumption of the comparators and the A/D converting device.

Further, a fourth aspect of the present invention relates to ananalog/digital converting device. The analog/digital converting devicecomprises: an A/D converter including multiple comparators each of whichcompares input analog signals with a predetermined reference voltage,for converting an input analog signal into a digital signal with apredetermined number of bits using comparison results from the multiplecomparators; a determining unit for determining which range the voltageof the input analog signal belongs to, before the comparators comparingthe input analog signal with the reference voltages, with regard tomultiple ranges into which the conceivable range of the voltage of theinput analog signal is divided with predetermined thresholds asboundaries; and a switching unit for changing the reference voltagesinput to the multiple comparators corresponding to the range determinedby the determining unit.

The aforementioned A/D converter may include a smaller number ofcomparators than those required for forming an A/D converter having afunction for converting the input analog signal into the digital signalwith the predetermined number of bits, with at least one of the multiplecomparators being operated so as to compare the input analog signal withmultiple kinds of reference voltages by switching the reference voltage.With such a configuration, the aforementioned reference voltages may bechanged such that the range thus determined by the aforementioneddetermining unit is included within a range between the maximum valueand the minimum value of the reference voltages input to theaforementioned multiple comparators. That is to say, an arrangement maybe made wherein the determining unit roughly determines which range thevoltage of the input analog signal belongs to, and the referencevoltages are changed such that the input analog signal in the range thusdetermined is appropriately converted into the digital signal. Such aconfiguration enables reduction of the number of the comparators,thereby reducing the circuit area while reducing power consumption.

The aforementioned analog/digital converting device may comprisemultiple conversion units each of which includes: the A/D converter; aD/A converter for converting the output from the A/D converter into theanalog signal; a subtracter for subtracting the output signal from theD/A converter, from the input analog signal; and an amplifier foramplifying the output signal from the subtracter, with the determiningunit being provided for each of the conversion units. The conversionunit may serve as a unit component of a pipeline A/D converter.

At least one of the multiple conversion units may include a switchingunit for switching whether the output signal from the amplifier withinthe same conversion unit is input as the input analog signal, or theoutput signal from a circuit other than the conversion unit is input asthe input analog signal. The conversion unit may serve as a unitcomponent of a cyclic A/D converter.

In the event that the output signal from the amplifier of the upstreamconversion unit is used as the input analog signal input to theconversion unit, the determining unit may acquire the output signal fromthe amplifier, and determine which range the output signal belongs to,before stable output of the output signal at an almost constant value.Such a configuration allows determination which range the voltage of theoutput signal from the amplifier, i.e., the voltage of the input analogsignal, belongs to, at an early stage, thereby allowing selection of thecomparators which are to be operated before input of signals to thecomparators, in order to control power supply. Furthermore, anarrangement may be made wherein the output signals are input to thecomparators after stable output of the output signal from the amplifier.

Further, a fifth aspect of the present invention relates to adetermining device. The determining device determines which range thevoltage of the input analog signal belongs to beforehand by comparingthe input analog signal with predetermined thresholds, from multipleranges into which the conceivable range of the voltage of the inputanalog signal input to a circuit other than the determining device isdivided with the thresholds as boundaries. With such a configuration,the circuit for processing the input analog signals acquires the rangewhich the voltage of the input analog signal belongs to beforehand,thereby enabling suitable processing corresponding to the range.

Further, a sixth aspect of the present invention relates to a method forcomparing analog signals. The comparison method comprises: determiningwhich range the voltage of the input analog signal belongs to, beforemultiple comparators comparing the input analog signal with referencevoltages, with regard to multiple ranges into which the conceivablerange of the voltage of the input analog signal is divided with thethresholds as boundaries; and controlling power supply for the multiplecomparators corresponding to the range determined in the determiningstep.

Further, a seventh aspect of the present invention relates to a methodfor comparing analog signals. The comparison method comprises:determining which range the voltage of the input analog signal belongsto, before multiple comparators comparing the input analog signal withreference voltages, with regard to multiple ranges into which theconceivable range of the voltage of the input analog signal is dividedwith the thresholds as boundaries; and changing the reference voltagesinput to the multiple comparators corresponding to range determined inthe determining step.

Moreover, this summary of the invention does not necessarily describeall necessary features so that the invention may also be sub-combinationof these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram which shows a configuration of an A/D convertingdevice according to a first embodiment;

FIG. 2 is a diagram which shows another example of the configuration ofthe A/D converting device according to the first embodiment;

FIG. 3 is a diagram which shows an internal configuration of the A/Dconverter according to the first embodiment;

FIG. 4 is a diagram which shows change in the output signal from asecond amplifier over time;

FIG. 5 is a time chart which shows action steps of a first sub-A/Dconversion unit;

FIG. 6 is a time chart which shows action steps of a second sub-A/Dconversion unit and a third sub-A/D conversion unit;

FIG. 7 is a diagram which shows a configuration of an A/D convertingdevice according to a second embodiment;

FIG. 8 is a diagram which shows another configuration of the A/Dconverting device according to the second embodiment; and

FIG. 9 is a diagram which shows an internal configuration of the A/Dconverter according to the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on preferred embodiments whichdo not intend to limit the scope of the present invention but exemplifythe invention. All of the features and the combinations thereofdescribed in the embodiments are not necessarily essential to theinvention.

First Embodiment

An A/D converting device including comparing units according to a firstembodiment has a configuration wherein the range, which the voltage ofan analog signal input to each sub-A/D converter belongs to, isdetermined beforehand, and power is supplied to only the comparatorsnecessary for this particular A/D conversion for converting the analogsignal into the digital signal; the analog signal being in the rangedetermined in the aforementioned determination processing, therebyreducing power consumption of the comparing units included within thesub-A/D converters. Thus, such a configuration enables reduction ofpower consumption of the comparing units and the A/D converting device.

FIG. 1 shows a configuration of an A/D converting device 1 according tothe first embodiment. The A/D converting device 1 has a configurationwherein sub-A/D conversion units 3, 4, 5, and 6, are connected inseries. The first sub-A/D conversion unit 3 includes a first amplifier10, an A/D converter 12, a D/A converter 13, a subtracter 14, and asecond amplifier 15. Each of the second sub-A/D conversion unit 4 andthe third sub-A/D conversion unit 5 includes a first amplifier 11, andalso the A/D converter 12, D/A converter 13, subtracter 14, and secondamplifier 15. The final sub-A/D conversion unit 6 includes only the A/Dconverter 12. Note that while FIG. 1 shows an example of a single-endedcircuit configuration for simplification, an arrangement may be madewith a differential circuit configuration.

The analog signal Vin input to an input terminal 18 is input to the A/Dconverter 12 of the first sub-A/D conversion unit 3, and is convertedinto a digital signal with a predetermined number of bits. In theexample shown in FIG. 1, the first A/D converter 12 outputs 4-bitdigital signals. The digital signals are output to a digital outputcircuit 17 and the D/A converter 13. The D/A converter 13 converts thedigital signal output from the A/D converter 12 into an analog signal.The first amplifier 10 samples and holds the input analog signal Vin,and outputs the signal to the subtracter 14. The subtracter 14 subtractsthe analog signal output from the D/A converter 13, from the analogsignal output from the first amplifier 10. The second amplifier 15amplifies the analog signal output from the subtracter 14 with anamplification factor of 2. Note that the amplification factor of thesecond amplifier 15 is not restricted to 2; rather, an amplifier withany desired amplification factor may be employed as the second amplifier15. The output signal from the second amplifier 15 is input to the firstamplifier 11 and the A/D converter 12 of the downstream sub-A/Dconversion unit 4.

The A/D converter 12 of the second sub-A/D conversion unit 4 outputs2-bit digital signals to the digital output circuit 17. The D/Aconverter 13 converts the digital signal output from the A/D converter12, into the analog signal, amplifies the analog signal with anamplification factor of 2, and outputs the amplified signal to thesubtracter 14. The first amplifier 11 samples and holds the input analogsignal, amplifies the analog signal with an amplification factor of 2,and outputs the amplified signal to the subtracter 14. Note that theamplification factor of the first amplifier 11 and the D/A converter 13is not restricted to 2, and that an amplifier and a D/A converter with adesired amplification factor may be employed as the first amplifier 11and the D/A converter 13, respectively. Furthermore, an arrangement maybe made without the first amplifier 11. With such a configuration, theD/A converter 13 outputs the analog signal to the subtracter 14 with anamplification factor of 1. For example, with a configuration wherein theA/D converter 12 acquires 2-bit digital signals, an arrangement ispreferably employed wherein the analog signal is amplified with anamplification factor of 4 by actions of the first amplifier 11 and thesecond amplifier 15.

The third sub-A/D conversion unit 5 acquires two-bit digital signals inthe same way as with the second sub-A/D conversion unit 4. Subsequently,the final sub-A/D conversion unit 6 acquires two-bit signals by actionsof the A/D converter 12, whereby conversion of the input analog signalinto the digital signal is performed. The digital output circuit 17outputs a 10-bit digital signal wherein the digital signals acquired bythe four sub-A/D conversion units are totaled up.

Next, description will be made regarding the functions of a determiningunit 20. An conceivable voltage range of the input analog signal inputto the sub-A/D conversion unit is divided into multiple ranges withpredetermined thresholds as boundaries beforehand. A determining unit 20determines which range the voltage of the input analog signal belongsto. With the present embodiment, the determining unit 20 is realized inthe form of a comparator. That is to say, the determining unit 20compares the input analog signal with a predetermined reference voltageso as to determine whether the voltage of the input analog signalbelongs to a higher range than the reference voltage, or belongs to alower range. A control unit 22 controls power supply for multiplecomparators included in the A/D converter 12 based upon the voltagerange of the input analog signal determined by the determining unit 20.Detailed description will be made later regarding the operations of thedetermining unit 20 and the control unit 22.

While FIG. 1 shows an example wherein each of the sub-A/D conversionunits 3, 4, 5, and 6 includes the determining unit 20, an arrangementmay be made wherein at least one of the sub-A/D conversion units 3, 4,5, and 6 include the determining unit 20. With such a configuration asshown in FIG. 1 wherein each of the sub-A/D conversion units 3, 4, 5,and 6 includes the determining unit 20, the range of the input analogsignal input to each sub-A/D conversion unit is appropriately determinedso as to control power supply for the comparators included in the A/Dconverter 12, thereby effectively reducing power consumption. Also,while FIG. 1 shows an example wherein each of the A/D conversion units3, 4, 5, and 6, includes the control unit 22, an arrangement may be madewherein only the A/D conversion units having the determining unit 20include the control unit 22. Furthermore, an arrangement may be madewherein the single control unit 22 controls power supply for thecomparators included in the multiple or all the A/D converters 12.

FIG. 2 shows another arrangement of the A/D converting device accordingto the first embodiment. An A/D converting device 2 has a configurationwherein a sub-A/D conversion units 3 and 7 are connected in series. Thefirst sub-A/D conversion unit 3 includes the first amplifier 10, A/Dconverter 12, D/A converter 13, subtracter 14, and second amplifier 15.The second sub-A/D conversion unit 7 includes the first amplifier 11,A/D converter 12, D/A converter 13, subtracter 14, second amplifier 15,and switches SW1 and SW2. While FIG. 2 shows an example of asingle-ended circuit configuration for simplification, an arrangementmay be made with a differential circuit configuration.

The sub-A/D conversion unit 3 converts the analog signal Vin input to aninput terminal 18 into the digital signal with a predetermined number ofbits in the same way as described above with reference to FIG. 1, andthe output signal from the second amplifier 15 is input to thedownstream sub-A/D conversion unit 7. The sub-A/D conversion unit 7 hasthe switches SW1 and SW2 for switching whether the output signal fromthe second amplifier 15 of the sub-A/D conversion unit 7 is to be usedas the input analog signal for the A/D converter 12, i.e., to be inputto the A/D converter 12 as a feedback signal, or the output signal fromthe second amplifier 15 of the upstream sub-A/D conversion unit 3 is tobe input to the A/D converter 12. First, the SW1 is turned on, the SW2is turned off, and the output signal from the second amplifier 15 of theupstream sub-A/D conversion unit 3 is input to the A/D converter 12,whereby the A/D converter 12 acquires the digital signal. In subsequentA/D conversions, the SW2 is turned on, the SW1 is turned off, and theoutput signal from the second amplifier 15 is input to the A/D converter12 as a feedback signal. With such a configuration, A/D conversion isperformed with the same single sub-A/D conversion unit 7 multiple times,thereby acquiring the following lower-bit digital signals. The sub-A/Dconversion unit 7 having such a loop configuration allows the user toadjust the number of the A/D-conversion loops so as to realize varioustypes and levels of performance, thereby improving degree-of-freedomfrom the perspective of circuit design.

FIG. 3 shows an internal configuration of the A/D converter 12 accordingto the first embodiment. The A/D converter 12 includes a comparing unit34 having multiple comparators 30 a through 30 h, and resistors R forcreating reference voltages input to the comparators 30 a through 30 h.FIG. 3 shows an example of the A/D converter 12 for outputting 3-bitdigital signals, which includes eight comparators 30 a through 30 h andeight resistors R. The eight resistors R each have the same resistancevalue so as to divide the potential difference between the maximum valueVRT and the minimum value VRB of the input analog signal into eightequal voltages, and these voltages thus created are input to thecomparators 30 a through 30 h as the reference voltages. With such aconfiguration, determination is made which range of these eight voltageranges the input analog signal belongs to, using the eight comparators30 a through 30 h. The determination result is converted into thedigital signal, and is output to the digital output circuit 17.

With the present embodiment, the determining unit 20 determinesbeforehand whether the input analog signal input to the A/D converter 12is higher or lower than the median of the maximum value VRT and theminimum value VRB. In a case wherein the input analog signal is higherthan the median, what the results of comparison would be regarding thecomparators 30 a through 30 d, each of which receives the referencevoltage lower than the median, is already obvious, and accordingly,power supply is not required for the comparators 30 a through 30 d.Accordingly, in this case, the control unit 22 reduces or stops powersupply for the comparators 30 a through 30 d. This enables reduction ofpower consumption of the comparing unit 34. On the other hand, in a casewherein the input analog signal is lower than the median, the controlunit 22 reduces or stops power supply for the comparators 30 e through30 h each of which receives the reference voltage higher than themedian.

With the A/D converting device having a differential analog circuit, thedetermining unit 20 compares the positive and negative output signalsVout+ and Vout− output from the upstream second amplifier 15 regardingmagnitude, as shown in FIG. 3. With the A/D converting device having asingle-ended analog circuit, the median of the maximum voltage and theminimum voltage is preferably input as a reference voltage to thecomparator forming the determining unit 20. As described above,determination whether the input analog voltage belongs to an upper rangeor a lower range with the median as a threshold is equivalent todetermination of the uppermost bit of the digital signal which is to beobtained using the A/D converter 12.

As described above, with the comparing unit 34 formed of the multiplecomparators 30 a through 30 h, the comparators 30 a through 30 h aredivided into two groups. With such a configuration, power is supplied tothe comparator group which has been determined to be operated based uponthe conceivable range of the input signal. FIG. 3 shows an examplewherein each of the comparators 30 a through 30 h includes a signal linefor receiving a control signal for controlling power supply for each ofthe comparators 30 a through 30 h. This enables switching of thecomparator group which is to be operated, corresponding to the range ofthe voltage of the input signal. As an another example, an arrangementmay be made wherein the control unit 22 directly controls the voltage orelectric current applied to the power line for supplying power to eachof the comparators 30 a through 30 h.

Furthermore, an arrangement may be made wherein the determining unit 20includes two or more comparators for determining the range of thevoltage of the input analog signal with a greater number of smallerranges. In this case, an arrangement may be made wherein the controlunit 22 reduces or stops power supply for those of the comparators 30 athrough 30 h, each of which receives the reference voltage outside ofthe range thus determined beforehand. Furthermore, the comparing unit 34may include the determining unit 20 or the control unit 22 therewithin.In this case, the control unit 22 may be formed of a switching devicefor turning on and off power supply for the comparators 30 a through 30h.

FIG. 4 shows change in the output signal from the second amplifier 15over time. In the drawing, at the time t1, signals are input to theinverting input terminal and the non-inverting input terminal of thesecond amplifier 15. Subsequently, the difference therebetween increasesfrom 0 at t1 to the stable state at t3. At the time t2, the secondamplifier 15 does not yet output a stable signal which is constant overtime. However, at this time, the output signal is sufficient fordetermining which signal of Vout+ and Vout− is greater than the other.Accordingly, the determining unit 20 determines the range of the outputsignal at this point in time. Subsequently, at the time t3, the secondamplifier 15 outputs sufficiently stable output signal, and the outputsignal is input to the downstream A/D converter 12.

FIG. 5 is a time chart which shows action steps of the first sub-A/Dconversion unit 3. The first amplifier 10 and the first A/D converter 12sample the input analog signal Vin, synchronously with the leading edgeof the clock signal CLK. At the same time, the determining unit 20samples the input analog signal Vin. Following sampling of the inputanalog signal Vin, the determining unit 20 compares the input analogsignal Vin with the reference voltage, and outputs comparison results tothe control unit 22. The control unit 22 transmits control signals tothe A/D converter 12 for controlling power supply for the comparators 30a through 30 h included in the A/D converter 12. The A/D converter 12awaits the control signals from the control unit 22. Upon reception ofthe control signals, the A/D converter 12 operates those of thecomparators 30 a through 30 h which are required for this particular A/Dconversion, whereby A/D conversion is performed. The first amplifier 10holds the sampled input analog signal Vin. The determining unit 20, theA/D converter 12, and the first amplifier 10, perform auto-zerooperation, synchronously with the trailing edge of the clock signal CLK.

FIG. 6 is a time chart which shows action steps of the second sub-A/Dconversion unit 4 and the third sub-A/D conversion unit 5. Thedetermining unit 20 samples the output signal from the second amplifier15 before the second amplifier 15 outputting the stable signal constantover time, and compares the sampled output signal with the referencevoltage, as shown in FIG. 4. The comparison results are transmitted tothe A/D converter 12 through the control unit 22 before the timing foroperating the A/D converter 12. The first amplifier 11 and the first A/Dconverter 12 sample the output signal from the second amplifier 15 ofthe upstream sub-A/D conversion unit, synchronously with the leadingedge of the clock signal CLK. At this time, the A/D converter 12 hasalready received the control signals from the control unit 22. The A/Dconverter 12 operates those of the comparators 30 a through 30 h whichare required for this particular A/D conversion, whereby A/D conversionis performed. The first amplifier 10 amplifies the sampled analog signalwith an amplification factor of 2. The determining unit 20 performsauto-zero operation, synchronously with the leading edge of the clocksignal CLK. The A/D converter 12 and the first amplifier 11 performauto-zero operation, synchronously with the trailing edge of the clocksignal CLK. The action steps for the final sub-A/D conversion unit 6 arethe same as shown in the time chart in the drawing, except for those ofthe first amplifier 11. As described above, the determining unit 20determines the range of the input signal prior to the timing foroperating the comparing unit 34 of the A/D converter 12, therebyenabling higher-speed A/D conversion.

Second Embodiment

With a second embodiment, the reference voltages input to comparatorsforming a comparing unit of an A/D converter are adjusted correspondingto the range which the voltage of the input analog signal belongs to,and which has been determined by the determining unit. This allowscircuit design with a reduced number of comparators, thereby allowingcircuit design with a reduced circuit area while reducing powerconsumption.

FIG. 7 shows an A/D converting device 41 according to the secondembodiment. Comparing the components of the A/D converting device 41with those of the A/D converting device 1 according to the firstembodiment shown in FIG. 1, the A/D converting device 41 includesswitching units 24 instead of the control units 22, and A/D converters19 instead of the A/D converters 12. The other components and operationof the A/D converting device 41 are the same as those according to thefirst embodiment, and the same components are denoted by the samereference numerals. The switching unit 24 adjusts the reference voltagesinput to the comparators of the A/D converter 19 corresponding to therange of the voltage of the input analog signal determined by thedetermining unit 20.

FIG. 8 shows a configuration of an A/D converting device 42 according tothe second embodiment. Comparing the components of the A/D convertingdevice 42 with those of the A/D converting device 2 according to thefirst embodiment shown in FIG. 2, the A/D converting device 42 includesswitching units 24 instead of the control units 22, and A/D converters19 instead of the A/D converters 12. The other components and operationof the A/D converting device 42 are the same as those according to thefirst embodiment, and the same components are denoted by the samereference numerals. The switching unit 24 adjusts the reference voltagesinput to the comparators of the A/D converter 19 corresponding to therange of the voltage of the input analog signal determined by thedetermining unit 20.

FIG. 9 shows an internal configuration of the A/D converter 19 accordingto the second embodiment. Comparing the components of the A/D converter19 with those of the A/D converter 12 according to the first embodimentshown in FIG. 3, the A/D converter 19 includes switches 32 a through 32h for switching the reference voltages input to the comparators 30 athrough 30 d, which is formed of a half number of the comparators ascompared with those according to the first embodiment. That is to say,the comparing unit 34 of the A/D converter 19 does not include all eightcomparators necessary for forming an A/D converter having a function forconverting analog signals into the 3-bit digital signals, i.e., includesfewer comparators. However, such a configuration wherein the referencevoltages are switched allows each comparator to compare the input analogsignal with multiple kinds of reference voltages, thereby enablingcircuit design with half as many comparators.

Specifically, in the event that the determining unit 20 has determinedthat Vout+ is greater than Vout− regarding the output signals from theupstream second amplifier 15, the switching unit 24 turns on theswitches 32 e through 32 h, and turns off the switches 32 a through 32d. As a result, the comparators 30 a through 30 d receives referencevoltages in the range A corresponding to the case wherein Vout+ isgreater than Vout−. On the other hand, in the event that the determiningunit 20 has determined that Vout+ is smaller than Vout− regarding theoutput signals from the upstream second amplifier 15, the switching unit24 turns off the switches 32 e through 32 h, and turns on the switches32 a through 32 d. As a result, the comparators 30 a through 30 dreceives reference voltages in the range B. That is to say, thecomparator 30 a has functions serving as the comparators 30 a and 30 eaccording to the first embodiment, for example.

Furthermore, an arrangement may be made wherein the determining unit 20includes two or more comparators for determining the range of thevoltage of the input analog signal with a greater number of ranges eachof which is smaller. With such a configuration, the switching unit 24switches the reference voltages input to the comparators 30 such thatthe range between the maximum value and the minimum value of thereference voltages input to the multiple comparators covers the rangedetermined by the determining unit 20.

As described above, description has been made regarding the presentinvention with reference to the aforementioned embodiments. Theabove-described embodiments have been described for exemplary purposesonly, and are by no means intended to be interpreted restrictively.Rather, it can be readily conceived by those skilled in this art thatvarious modifications may be made by making various combinations of theaforementioned components or the aforementioned processing, which arealso encompassed in the technical scope of the present invention.

1-2. (canceled)
 3. A comparing device, comprising a plurality ofcomparators for comparing input analog signals with predeterminedreference voltages, wherein the conceivable range of the voltage of saidinput analog signal is divided into a plurality of ranges withpredetermined thresholds as boundaries, and wherein said referencevoltages, which belong to different ranges, are changed such that thereference voltage belonging to the range which the voltage of said inputanalog signal belongs to is input to said plurality of comparators.
 4. Acomparing device according to claim 3, further comprising: a determiningunit for determining which range the voltage of said input analog signalbelongs to; and a switching unit for changing said reference voltagesinput to said plurality of comparators corresponding to the rangedetermined by said determining unit, wherein said switching unit changessaid reference voltages such that the range between the maximum valueand the minimum value of said reference voltages input to said pluralityof comparators covers said range determined by said determining unit. 5.(canceled)
 6. An analog/digital converting device comprising: an A/Dconverter including a plurality of comparators each of which comparesinput analog signals with a predetermined reference voltage, forconverting an input analog signal into a digital signal with apredetermined number of bits using comparison results from saidplurality of comparators; a determining unit for determining which rangethe voltage of said input analog signal belongs to, before saidcomparators comparing said input analog signal with said referencevoltages, with regard to a plurality of ranges into which theconceivable range of the voltage of said input analog signal is dividedwith predetermined thresholds as boundaries; and a switching unit forchanging said reference voltages, which belong to different ranges, suchthat the reference voltage belonging to the range determined by saiddetermining unit it input to said plurality of comparators.
 7. Ananalog/digital converting device according to claim 6, wherein said A/Dconverter includes a smaller number of comparators than those requiredfor forming an A/D converter having a function for converting said inputanalog signal into said digital signal with the predetermined number ofbits, and wherein at least one of said plurality of comparators isoperated so as to compare said input analog signal with plurality kindsof reference voltages by switching said reference voltage.
 8. (canceled)9. An analog/digital converting device according to claim 6, comprisinga plurality of conversion units each of which includes: said A/Dconverter; a D/A converter for converting the output from said A/Dconverter into the analog signal; a subtracter for subtracting theoutput signal from said D/A converter, from said input analog signal;and an amplifier for amplifying the output signal from said subtracter,wherein said determining unit is provided for each of said conversionunits.
 10. An analog/digital converting device according to claim 7,comprising a plurality of conversion units each of which includes: saidA/D converter; a D/A converter for converting the output from said A/Dconverter into the analog signal; a subtracter for subtracting theoutput signal from said D/A converter, from said input analog signal;and an amplifier for amplifying the output signal from said subtracter,wherein said determining unit is provided for each of said conversionunits.
 11. (canceled)
 12. An analog/digital converting device accordingto claim 9, wherein at least one of said plurality of conversion unitsincludes a switching unit for switching whether the output signal fromsaid amplifier within the same conversion unit is input as said inputanalog signal, or the output signal from a circuit other than theconversion unit is input as said input analog signal.
 13. Ananalog/digital converting device according to claim 10, wherein at leastone of said plurality of conversion units includes a switching unit forswitching whether the output signal from said amplifier within the sameconversion unit is input as said input analog signal, or the outputsignal from a circuit other than the conversion unit is input as saidinput analog signal. 14-19. (canceled)
 20. A comparison method foranalog signals comprising: determining which range the voltage of theinput analog signal belongs to, before a plurality of comparatorscomparing said input analog signal with reference voltages, with regardto a plurality of ranges into which the conceivable range of the voltageof said input analog signal is divided with said thresholds asboundaries; and changing said reference voltages, which belong todifferent ranges, such that the reference voltage belonging to the rangedetermined in said determining is input to said plurality ofcomparators.